Digital communication systems, such as selective call paging systems, have utilized binary digital signalling formats for many years. When such digital signalling formats have been utilized, the digital information has been modulated onto the carrier using modulation techniques such as frequency shift keying (FSK), wherein the digital information is directly modulated onto the carrier. Within the receiver, the frequency shift keyed carrier is demodulated and further processed to provide a stream of digital data. Such modulation and demodulation techniques, while effective at lower data bit rates, such as data bit rates below approximately 6000 bits per second, are not effective at higher data bit rates due to synchronization problems in simulcast systems. As a result, other modulation techniques are required to enable higher data throughput at lower symbol rates.
One such modulation technique which allows higher data throughput at lower symbol rates is a multi-level frequency modulation technique, such as four-level frequency modulation (FM). As compared to conventional frequency shift keyed modulation, four-level FM allows twice the data throughput for a given data symbol rate. In conventional four-level FM receivers, the carrier is received and converted to analog voltages. The analog voltages then are compared to pre-programmed threshold levels to allow a determination of which of four data symbols should be generated for each analog voltage. Although this demodulation technique provides a stream of digital data, the data may not always be recovered accurately because the pre-programmed threshold levels do not allow for variations in the received carrier which result in voltage offsets. As further offsets in the received carrier occur, greater voltage offsets are encountered and incorrect data symbols can be generated, possibly causing erroneous recovery of information from the radio signal. Thus, what is needed is a method and apparatus, for use with multi-level signalling formats, for dynamically varying the threshold levels utilized for recovery of data symbols in response to variations in the voltage offsets caused by voltage variations in the received carrier.